This invention relates to a magnetic reproducing apparatus provided with a limiter circuit for obtaining a reproduced luminace signal used in a magnetic reproducing apparatus such as a video tape recorder (VTR), etc.
In VTRs for consumer use generally and widely used at present, at the time of recording, a luminance signal (Y signal) is frequency-modulated, resulting in a FM luminance signal. At this time, a carrier chrominance signal (C signal) is subjected to frequency conversion so that it has a frequency lower than that of the FM luminance signal resulting in a low frequency carrier chrominance signal. At the time of reproducing a magnetic tape on which a multiplexed signal obtained by applying frequency-division multiplexing to these FM luminance signal and low frequency carrier chrominance signal is recorded, an approach is employed to separate the FM luminance signal and the low frequency carrier chrominance signal from the multiplexed signal to demodulate the FM luminance signal to obtain the original Y signal, and to apply frequency conversion to the low frequency carrier chrominance signal so that it has a frequency equal to the original frequency to obtain the original C signal.
For separating an FM luminance signal from the multiplexed signal recorded on the magnetic tape to reproduce an original Y signal, the following technique is employed.
Namely, the multiplexed signal reproduced alternately, e.g., by two video heads scanning on a magnetic tape is amplified at the reproducing preamplifier circuits of the two systems, and is then delivered to a high-pass filter circuit through a switch for switching two outputs from the preamplifier circuits. At this filter circuit, the low frequency carrier chrominance signal is separated and removed. The FM luminance signal thus obtained is delivered to a drop-out compensation circuit, at which compensation of drop-out is made. Then, this signal is applied to the limiter circuit, at which level change is removed. Then, the signal thus obtained is delivered to the FM demodulation circuit and is demodulated thereat. Thus, the Y signal which has been subjected to video emphasis can be obtained.
Reversal from white peak to black (hereinafter referred to as reversal of white peak) is known as a phenomenon to impede or obstruct an improvement in the reproduced picture quality. For a limiter circuit for preventing occurrence of such an unfavorable phenomenon, a double limiter circuit is used. This double limiter circuit serves to apply double-limiting operation to a drop-out compensated FM luminance signal delivered thereto to prevent an inverted white peak to sufficiently effect a video emphasis, thus to improve S/N ratio of the FM luminance signal before demodulation.
FIG. 1 is a block diagram of the double limiter circuit used for a conventional magnetic reproducing apparatus. FIGS. 2A to 2C show signal waveforms in a recording system, FIG. 2A is of a luminance signal to be recorded, FIG. 2B is of the luminance signal subjected to video pre-emphasis and FIG. 2C is of the FM luminance signal to be recorded. FIGS. 2D and 2F through 2J show signal waveforms of respective constituent portions of the double limiter circuit shown in FIG. 1. And FIG. 2E shows a waveform of a signal obtained by passing a reproduced FM luminance signal through a sole limiter circuit in which carrier waves at the white peak are lost result in serious reversal of white peak.
The double limiter circuit 1 used for a conventional magnetic reproducing apparatus comprises, as shown in FIG. 1, an input terminal 2 to which an FM luminance signal (signal shown in FIG. 2D) from a drop-out compensation circuit (not shown) is applied, a high-pass filter circuit 3 responsive to the FM luminance signal applied thereto through the input terminal 2 to output a signal (signal shown in FIG. 2F) corresponding to the medium and high frequency band of the FM luminance signal, in which the lower side-band component is eliminated from the FM luminance signal, a low-pass filter circuit 4 responsive to the FM luminance signal applied thereto through the input terminal 2 to derive, from the FM luminance signal, a signal having the lower side-band component (signal shown in FIG. 2H) to output it therefrom, a first limiter circuit 5 responsive to the output signal delivered thereto to output a signal (signal shown in FIG. 2G) obtained by allowing the amplitude of the signal to be made uniform and removing noises in the vicinity of the carrier of the signal, an adder circuit 6 for outputting an addition signal (signal shown in FIG. 2I) obtained by adding an output signal from the low-pass filter circuit 4 to an output signal from the first limiter circuit 5, a second limiter circuit 7 responsive to the addition signal from the adder circuit 6 to output an FM luminance signal (signal shown in FIG. 2J) obtained by allowing the amplitude of the addition signal to be made uniform, and an output terminal 8 for delivering, to an FM demodulation circuit (not shown), the FM luminance signal from the second limiter circuit 7.
As stated above, such a conventional reproducing apparatus including double limiter circuit 1 can reduce more effectively the influence of the inverted white peak as compared to other conventional apparatus.
While the double limiter circuit 1 used in the above-described magnetic reproducing apparatus can reduce more effectively the influence of the reversal of white peak as compared to other conventional apparatus, it has the following drawbacks. First, it is difficult to make a phase matching or adjustment between the output signal from the first limiter circuit 5 and the output signal from the low-pass filter circuit 4. Secondly, when the FM luminance signal applied to the input terminal 2 is completely separated and outputted by the high-pass filter circuit 3 and the low-pass filter circuit 4, a demodulated reproduced luminance signal obtained by delivering the FM luminance signal from the output terminal 8 to the FM demodulation circuit (not shown) has a frequency characteristic which is not flat continuously from the low frequency band to the high frequency band.
For this reason, a filter circuit having a steep attenuation characteristic cannot be used as the high-pass filter circuit 3 and the low-pass filter 4. As a result, a signal having a lower side-band component is partially outputted from the high-pass filter circuit 3, so a carrier loss is produced at the first limiter circuit 5 of the next stage, lacking in necessary zero cross point. Accordingly, reversal of white peak is produced, thus failing to completely eliminate the reversal of white peak.